BR: add general commands for operator

[YuJuncen]

Enhancement

During implementing backing up via EBS volumes across k8s clusters, we need to control the gc finer -- because each BR can only access resources in one k8s cluster (Without changing more code in other components -- say, letting PD to synchronize the volume info).
Consequently, we are going to split out some atomic commands for operator to use. For example, the operator can manually pause the GC when backing up -- so it can run BR command in each k8s cluster and the GC won't be advanced.